Most edible oils go through refining operations comprising pretreatment steps known as degumming and/or neutralisation, most often followed by treatment with a solid adsorbent, e.g. acid activated clay, known as bleaching. Pretreatment of oils for non-edible use, such as manufacture of biodiesel, may or may not include the bleaching operation. The pretreated oil is subsequently subjected to a high temperature operation known as deodorization. Deodorisation is conducted under vacuum and consists in general of two main process steps, a “heat bleaching” step, typically conducted within a range from about 240 to about 270° C. within a range of about 0.5 to about 1.5 hrs retention time and a second step consisting of stripping of volatiles by the use of steam. These steps may occur simultaneously or sequentially. Pretreatment of oil for non-edible use such as biodiesel manufacture does normally not include the heat bleaching step. Steam stripping, also known as de-acidification, of fats and oils, comprises reduction of the content of free fatty acid (FFA) and other volatiles by stripping with steam under vacuum. Volatiles are mainly those present in the pretreated oil, but volatiles may also be formed during the deodorisation operation, e.g. at the heat bleaching step. Steam stripping of volatile components may take place before, during or after the heat bleaching step, in any combination of those possibilities.
In recent years some refiners has in their deodorization sections included an additional condensation stage an arrangement often referred to as “double scrubbers” disclosed in U.S. Pat. No. 6,750,359, having an additional scrubber at high temperature. While use of this “double scrubber” enables withdrawal of a side-stream of condensate from the high temperature condensation step enriched in micronutrients, there remains a significant loss of micronutrients in the FFA rich overhead stream. A further significant limitation of such approach is that the separation efficiency in the hot scrubber is closely linked to the operating conditions of the deodorizer, where the main function is to produce edible oil by stripping steam under vacuum. Such linkage will impose rather low limits for the obtainable enrichment for the micronutrients in any distillate stream withdrawn from the hot scrubber. Other processes according to the prior art are disclosed by U.S. Pat. No. 7,598,407 and US2010/0200805. One example of deodorisation conducted by steam stripping in combination with heat bleaching is disclosed by WO 98/00484. Developments and improvements concerning deodorizations are disclosed in an article published at the Proceedings of the World Conference on Palm and Coconut Oils for the 21st Century, p. 67ff (1999) having the title “Recent Developments and Improvements in Palm Oils Stripping and Fatty Acid Destillation” by the author Peter Fassler.
Simultaneously with the removal of FFA, the steam stripping action will also partly remove valuable components such as micronutrients including tocopherols, sterols, squalene as well as “neutral oils”, i.e. tri-, di- and mono-acylglycerides (TAG, DAG, MAG), which represents a loss of valuable main product. Volatiles are condensed in a cold condensation stage, before the stripping steam along with non-condensable gases (e.g. inleakage air) are being lead to the vacuum system. Such a cold condensation stage commonly operates within a range from about 40 to about 60° C. and is commonly implemented as a scrubber loop, where cold distillate is used to condense the volatiles. The cold distillate will, besides the FFA:s, also contain micronutrients and “neutral oils”. Due to the high value of the micronutrients there is particular interest in recovering those from the distillate. However, the micronutrients need to be enriched in their concentration in the plant so that their further processing in purification plants and transport to such plants can take place in an economical manner. The loss of “neutral oils” will further dilute any enriched stream of micronutrients that can be withdrawn from the system.
A current trend in the edible oil industry is to use certain enzymes, commonly known as phospholipase A (“PLA”) type enzymes, to enable removal of phosphorous containing components (“phospholipids”) of the crude edible oil down to very low ppm levels, acting by producing more water-soluble lysophospholipids by splitting off a fatty acid from the phospholipid. However, a side effect of this process is increased production of FFA, which will dilute the micronutrients in the deodorizer distillates. Another type of enzymes commonly known as phospholipase C (“PLC”) type enzymes have also recently been taken into large scale industrial use. These enzymes give increased yields of oil-product by converting phospholipids to DAG. However, the increased level of DAG in the oils tends to aggravate the problem with neutral oils loss during the steam stripping operation since the vapour pressure of DAG is higher than the vapour pressure of TAG.
When pretreating biodiesel it is of interest to apply high severity conditions i.e. high stripping temperature for instance within the range 265 to 275° C., high amount of stripping steam for instance within the range 1 to 2% relative to the amount of oil feed, and deep vacuum for instance within the range 1.5 to 2 mbar in order to recover as much of the micronutrients as possible. However, such high severity conditions will also lead to increased losses of neutral oil, which, besides being a value loss, also will tend to dilute micronutrients.